1. Field of the Invention
The present invention relates to a method for producing microcapsules, microcapsules obtained by the method, a recording material including the microcapsules, and a heat-sensitive recording material.
2. Description of the Related Art
Techniques of encapsulating a main active ingredient are utilized in wide areas such as heat-sensitive or pressure-sensitive recording materials, clothing fields (such as perfume capsules), medical fields (such as drug delivery systems), and food fields (such as artificial salmon roe). Such a microcapsule is generally formed in such a manner that the active ingredient diffuses to the out side of the capsule only when its separating function is lost by external action such as heating, light irradiation, pressure, pH, enzyme or hydration. Therefore, the desired function is exhibited only when such an operation is conducted.
A heat-sensitive recording material is a material which records an image by being heated with a thermal head. The heat-sensitive recording material has been popularized in a wide variety of fields because the heat-sensitive recording material is relatively inexpensive, and its recording device is simple, highly reliable, and maintenance-free. Under such circumstances, there has been an increasing demand in recent years for higher performance including high image quality and improvement in storage stability. Extensive studies have been conducted on color developing density, image quality and storage stability of the heat-sensitive recording material.
As the heat-sensitive recording material, a large number of heat-sensitive recording materials using diazonium salts as color-developing components have been developed for the purpose of fixation of recording images after thermal recording. An image is formed on the heat-sensitive recording material containing a diazonium salt by heat, and light is applied to the image so as to decompose a residual diazonium salt and to fix the image. When the diazonium salt is encapsulated, raw storage stability and color developing density can be improved significantly.
A method of forming a microcapsule is generally divided into a chemical technique, a physicochemical technique, and physical technique. Various specific methods have conventionally been proposed. Examples of the disclosed methods include: a method utilizing coacervation of a hydrophilic wall forming material (see, for example, U.S. Pat. No. 2,800,457); an interfacial polymerization method (see, for example, U.S. Pat. No. 3,287,154, U.K. Patent No. 990443, and Japanese Patent Publication (JP-B) No. 38-19574); a method using polymer precipitation (see, for example, U.S. Pat. No. 3,418,250); a method using an isocyanate polyol wall-forming material (see, for example, U.S. Pat. No. 3,796,669); a method using an isocyanate wall-forming material (see, for example, U.S. Pat. No. 3,914,511); a method using urea-formaldehyde-based and urea-formaldehyde-resorcinol-based wall forming materials (see, for example, U.S. Pat. No. 4,001,140); a method using a polymer including polyurethane-polyurea as a main component (see, for example, JP-B No. 04-75147); a method using a mixture of xylylene diisocyanate and polymethylene polyphenyl isocyanate (see, for example, JP-B No. 06-86154); a method of using a polyvalent isocyanate previously reacted partially with monoalcohol (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 5-317694); a method using wall forming materials such as melamine-formaldehyde resins, hydroxypropyl cellulose or the like, (see, for example, U.S. Pat. No. 4,025,455); an in situ method utilizing monomer polymerization (see, for example, JP-B No. 36-9168); an electrolytic dispersion cooling method (see, for example, U.K. Patent No. 952807); and a spray drying method (see, for example, U.S. Pat. No. 3,111,407).
In the interfacial polymerization method, an oil phase in which a core material is dissolved or dispersed in a hydrophobic organic solvent is poured into an aqueous phase containing a water-soluble polymer. This mixture was emulsified by using a homogenizer, followed by heating. Accordingly, a polymer forming reaction occurs at an oil-water interface. Consequently, a microcapsule wall made of a polymer substance is formed, and the core material is encapsulated. This interfacial polymerization method can provide a microcapsule whose storage stability is excellent and whose particle diameter is uniform, in a short period of time. Interfacial polymerization method having such advantages is extensively utilized.
In the in situ polymerization method, a capsule wall is formed by a polymerization film of a radical polymerizable monomer, and there are two cases: one case is a case where a polymer is deposited from the interior of a core material to form a capsule film; and the other case is a case where a polymer is deposited from the exterior of the core material to form a capsule film. As one of the characteristics of this method, a core material to be encapsulated is not limited to liquid, and solid or gas can be used in place of liquid.
In the in situ polymerization method, capsule walls are formed from polymer walls of radical-polymerizable monomers, and there is the case where the capsule film is formed by polymer deposition starting from the inside of the core substance or from the outside of the core substance, and there is an advantage that not only a liquid but also a solid or gas can be capsulated as the core substance.
In the conventional interfacial method and in situ polymerization method, however, there is a problem that when the core substance has a poor solubility in an oil phase solvent or a highly surface-active substance is contained, the emulsion polymerization reaction on the interface is easily inhibited, which results in gradual leakage of the core substance or in formation of secondary aggregates. When irregular microcapsules having such a poor inclusion ability are used in a heat-sensitive recording material, the substance in the microcapsule easily leaks out of the capsule to color the background area. Therefore, in that case, it is difficult to imagewise record an image.
In the field of a microcapsule used for a recording material, it is required to control the properties of a capsule in accordance with the target performance. Examples of the required properties of the capsule include: (1) excellent storage stability in a solution; (2) excellent long-term raw storage stability; (3) ability to suppress background fogging; (4) high transparency of material at heating; (5) stable high color development density; (6) less variation of heat sensitivity; (7) excellent light fastness or water resistance; (8) less yellowing or blemish; and (9) heat resistance or moisture resistance. For meeting these various needs, it is desirable that various wall materials can be used for forming microcapsules and the microcapsules are produced with the minimum restrictions and problems.
In the conventional methods, however, when a wall material is added to an oil phase which is to become a core substance, a necessary amount of the desired wall material cannot be added in some cases because of a change in solubility or deterioration in compatibility. Consequently, in such cases, the reaction of forming the capsule wall does not proceed, an the core substance cannot be sufficiently covered with the capsule wall. Particularly when a heterocyclic diazonium salt having an improved heat stability and a higher ability to donate an electron to a diazonio group is used, the solubility of the diazonium is in the oil phase is decreased owing to π electron-π electron stacking, compared to conventional aryl diazonium salts. This poor solubility causes a serious problem of a reduction in the degree of whiteness in the background area.
When the core substance and capsule wall material easily react with each other, they react with each other at mixing to cause fogging and coloring. Since heat is generated upon contact with water or high-speed stirring at emulsification, in some cases, the reaction proceeds partially to inhibit sufficient emulsification. There are also problems that after the capsule-forming reaction is completed, unreacted wall material remains in the core substance, and that the size of the capsule particle cannot be sufficiently reduced.
Because of the problems such as a limited range of selection of microcapsule wall materials for desired performance and lack of production suitability as described above, there are considerable restrictions on the wall material which can be actually formed into microcapsules. Therefore, there is a strong demand for a method for producing microcapsules having an arbitrary capsule characteristics.
Particularly when microcapsules are used in heat-sensitive recording materials and the like, there are various restrictions. There are problems such as a limited range of selection of microcapsule materials if the recording material has to have characteristics such as high sensitivity, excellent raw storage stability and background whiteness, and such microcapsules are not suitable for production. Therefore, there is strong demand for development of a new method for producing microcapsules by which microcapsules suitable for heat-sensitive recording materials and the like can be widely and freely designed. There are also demands for development of a microcapsule obtained by the method and a heat-sensitive recording material including the microcapsule.